1. Field of the Invention
The present invention relates to an end mill for finishing the face(s) or side(s) of hard work, having a plurality of helix cutting edges on its periphery.
2. Prior Art
An end mill shown in FIG. 2 has been conventionally used as a rotary cutting tool of a machine tool such as a milling machine to cut general-purpose materials such as steel. Generally, such an end mill has sharp cutting edges 2 disposed on an imaginary cylinder and having a helix angle .theta.a and a positive rake angle .theta.r. The end mill also has large flutes 25. This configuration facilitates the edges to cut into work and to allow chips to be discharged smoothly, ensuring very high cutting efficiency. The angles of the cutting edges 2 can be properly formed to have an optimum shape depending on the characteristics of the work. When an end mill with sharp cutting edges is used to cut hard work which cannot be cut efficiently, the edges are worn significantly and are apt to be chipped during cutting. To solve these problems, the specifications of the cutting edges 2 have been improved. More specifically, the strength of the cutting edges 2 are increased by decreasing the rake angle .theta.r and the relief angle .theta.c, and the rigidity of the end mill is also increased by decreasing the depths of the flutes 25, significantly preventing the cutting edges 2 from being worn and chipped.
It is known that an end mill with more cutting edges can have a longer service life, particularly when the end mill is used to cut hard work. Therefore, increasing the number of the cutting edges is also a means for improvement. In addition, the cutting conditions of the end mill, such as the depth of cut, are loosened and the end mill with improved specifications (increased number of cutting edges and other improvements in the performance of the edges) is used, when the end mill cannot efficiently cut the hard work.
To efficiently finish the face or side of the hard work, end mills must be made of materials with sufficient hardness and strength suited for cutting the hard work. High-speed steel and cemented carbide are generally used as the materials of end mills. When cutting the hard work, end mills made of cemented carbide and coated with hard substance such as TiN are used widely. Even in this case, however, such end mills with cutting edges having conventional shapes cannot cut work harder than HRC55.
The demand for finishing the face or side of hard work is increasing in many industrial fields. However, it is almost impossible to cut hard work such as hardened tool steel having a hardness of HRC60, no matter how the specifications such as the rake angle, relief angle and the number of the cutting edges of a conventional end mill are modified, because the modified specification values are not appropriate to practical use. This problem cannot be solved by changing cutting conditions. To cope with this situation, grinding and electrical discharge machining methods have been used. However, these methods are slow in cutting speed, resulting in low cutting efficiency.
Very hard materials such as ceramics, CBN (Cubic Boron Nitride) and diamond are used as cutting tool materials. Although these materials are extremely hard, they are relatively fragile and liable to be chipped during end mill cutting because cutting is performed intermittently. In addition, they cannot be formed into desired shapes due to low machinability. As a result, they are used only for limited applications, such as slight amounts of cutting.